Add doc explaining how language versioning and experiments work.
Add doc explaining how language versioning and experiments work.

Change-Id: I9c30bcda07321065ae215c2d3ee5029ed66e587e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138680
Auto-Submit: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Reviewed-by: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@google.com>
Commit-Queue: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk